1. Field of the Invention
This invention relates to methods of fabricating free-standing, frame-supported thin polymeric films.
2. Prior Art
Thin films of polymeric materials are used extensively in extreme ultraviolet (EUV) and x-ray optics. Some of the current and potential applications of thin polymeric films include bandpass filters and proportional counter entrance windows, two common elements of x-ray instrumentation.
By way of example, proportional counters are one type of detection device used in soft x-ray astronomy and microscopy. The entrance window of a gas scintillation type of proportional counter isolates the chamber housing the scintillation gas from the counter's surrounding environment. The scintillation gas, often xenon or a mixture of argon and methane, is maintained at atmospheric pressure. X-rays travel from the object under observation through the window and into the bulk of the scintillation gas. Events in the scintillation gas "count" the number of incoming photons.
Proportional counter windows have both optical and mechanical requirements to satisfy. The degree to which x-rays are attenuated by the window is an important factor to be considered in the design of the counter. The materials comprising the entrance window absorb x-rays according to the relation: EQU T=e.sup.-.alpha.x,
where T is the optical transmission of the film (a percentage, and therefore dimensionless), .alpha. is the linear absorption coefficient of the film material (in cm.sup.-1), and x is the film thickness (in cm). Because transmission depends inversely and exponentially on film thickness, absorption by the entrance window can be minimized by using the thinnest possible film that satisfies mechanical requirements. Absorption can also be minimized by choosing materials low in atomic number, such as carbon.
Proportional counters in x-ray astronomy must be utilized aboard orbiting spacecraft since x-rays are not transmitted by the earth's atmosphere. In this application the window must survive the vibrations accompanying launch of the spacecraft, and the pressure differential of one atmosphere generated by the vacuum of space. Typical windows "leak" gas. The scintillation gas can pass through pinholes in the film, and can diffuse directly through the bulk of the material itself. Gas leaks require the counter to be replenished by an exterior supply to compensate for the mass of gas which is lost to space.
The entrance window must be electrically conducting. This can be accomplished by mounting the window to a wire mesh, which serves as a mechanical support as well as an electrical conductor, or by metallizing the surface of the polymeric film using standard deposition techniques such as vacuum evaporation and sputtering.
To date, the materials most capable of fulfilling the optical and mechanical requirements of proportional counter entrance windows include polypropylene, polycarbonate, Formvar (TM), and elemental carbon. Chemically Formvar is poly(vinyl formal), and is sold by (among others) the Monsanto Company. Some investigators design filters composed of a single layer of material, using only one of these materials at a time, whereas others assemble composite structures with two or more layers of these materials in the same filter.
The use of a mesh supported, 1,000 .ANG. thick film of carbon as the entrance window for an x-ray microscope's proportional counter is discussed in Optical Systems for Soft X-Rays by A. G. Michette, p. 261, Plenum Publishing Corporation (1986). A 3.3 micron polypropylene entrance window, coated with colloidal carbon to provide electrical conductivity, is disclosed by M. Heppener and D. G. Simons in an article entitled "A Large Aperture Imaging Gas Scintillation Proportional Counter" in X-Ray Instrumentation in Astronomy II, Proc. SPIE, Vol. 982, pp. 139-146 (1988). A composite entrance window comprising a one micron polypropylene foil coated with carbon and Lexan (TM) is discussed in an article entitled "X-Ray Calibration of the ROSAT Position Sensitive Proportional Counter" by U. G. Briel, E. Pfeffermann, G. Hartner, and G. Hasinger in X-Ray Instrumentation in Astronomy II, Proc. SPIE, Vol. 982, pp. 401-408 (1988). Lexan is a form of polycarbonate which is owned by the General Electric Company.
In an article entitled "Design of High Performance Soft X-Ray Windows" in X-Ray/EUV Optics for Astronomy and Microscopy, Proc. SPIE, Vol. 1160, pp. 56-61 (1989), R. T. Perkins, D. D. Allred, L. V. Knight and J. M. Thorne teach that compound window materials containing a mixture of carbon, nitrogen, and oxygen would be superior to polypropylene, which contains essentially only carbon, because "single element windows are `colored` in that they transmit certain wavelengths better than others." A window with equal mixtures of carbon, nitrogen and oxygen would "transmit the wavelengths which correspond to each element roughly equivalently." Perkins et al. submit that "such windows would be approximately neutral density for elemental soft x-rays . . . , that such `gray windows` would be a noteworthy achievement . . . ," and suggest that " . . . one of the families of materials which contain carbon, nitrogen, and oxygen are the polyimides."
In the European Space Agency's Newsletter of the Astrophysics Division, No. 9, p. 3, (August, 1989), A. Smith discusses the use of a one micron polypropylene foil coated with Lexan to serve as an entrance window on the low energy gas scintillation proportional counter (LEGSPC) of the Italian Space Agency's X-Ray Astronomy Satellite (SAX). Smith also discloses an "alternative in the form of a polyimide foil," which should be "more robust, less leaky, and able to withstand much higher temperatures than polypropylene." Additionally, the "need for a gas supply system for the detector would be obviated."
In their article entitled "Soft X-Ray Windows for Position Sensitive Proportional Counters" in EUV, X-Ray and Gamma-Ray Instrumentation for Astronomy, Proc. SPIE, Vol. 1344, pp. 100-105 (1990), S. Nenonen, H. Sipila, P. Jalas and R. Mutikainen outline the development of polyimide windows 0.5 to 2.5 microns thick for the Soviet Spectrum-X-Gamma satellite. The polyimide was coated with aluminium to reduce gas permeation and the transmission of visible and infrared light.
By way of a second example of the role thin polymeric films play in x-ray optics, bandpass filters are routinely enlisted in EUV and x-ray astronomy to isolate a specific energy band. X-ray telescopes have been directed at the sun, for example, to study the distribution of the Fe IX and Fe X ions that emit between 171 and 175 .ANG. in the x-ray region of the spectrum, as described by A. B. C. Walker, J. F. Lindblom, R. H. O'Neal, M. J. Allen, T. W. Barbee, and R. B. Hoover in "Multi-Spectral Solar Telescope Array," Optical Engineering, Vol. 29, No. 6, pp. 581-591 (1990). Elemental carbon and carbon containing polymeric films are especially useful for blocking unwanted EUV radiation longer than 400 .ANG. in wavelength. Polymeric films are frequently coated with metals that serve to further define the bandpass of interest. Aluminium is a particular favorite for rejecting the visible light in solar radiation that occurs at 10.sup.10 times the energy in the x-ray region of the spectrum. If not eliminated, the visible light "bleaches," or overexposes the photographic film recording the x-ray image.
In addition to their role as filter materials, polymeric films can serve as a substrate on which to deposit the metallic layers. If this is the case, particulate contamination in the polymeric film is disasterous since it leads to the formation of pinholes which leak visible light. Particles interfere with the condensation and subsequent surface diffusion of metal atoms necessary for uniform growth of the metal film. The particles can also act as stress concentrators in films, eventually leading to the opening up of pinholes as a result of stress relaxation.
The use of the carbon containing compounds Lexan, phthalocyanine, and Parylene N (TM) in bandpass filters is discussed in an article entitled "Thin Film Filter Performance for Extreme Ultraviolet and X-Ray Applications" by F. R. Powell, P. W. Vedder, J. F. Lindblom and S. F. Powell in Optical Engineering, Vol. 29, No. 6, pp. 614-624 (1990). Chemically Parylene N is poly(p-xylylene), which is owned by the Union Carbide Corporation. E. Spiller, K. Grebe, and L. Golub report work relating to polypropylene/carbon composite filters in "Filters for Soft X-Ray Solar Telescopes," Optical Engineering, Vol. 29, No. 6, pp. 625-631 (1990).
Polyimides have not been discussed in the literature pertaining to bandpass filters. The mechanical properties of polyimides relative to polypropylene and polycarbonate, however, has been well covered in the prior art. The strength of these polymers when processed by injection molding has been reviewed by R. B. Seymour and C. E. Carraher in their book Polymer Chemistry, published by Marcel Dekker, Inc., p. 451 (1981):
______________________________________ Tensile Flexural Strength Strength (kg/cm.sup.2) (kg/cm.sup.2) ______________________________________ polypropylene 343 490 polycarbonate 630 945 polyimide 1190 1100 ______________________________________
The table suggests that for films of equivalent strength, a polyimide filter could be constructed thinner than one of either polypropylene or polycarbonate, and, therefore, the polyimide filter would transmit more of the radiation impinging thereon. The temperature stability of polyimides relative to polypropylene and polycarbonate, furthermore, indicates that polyimide films could be more easily metallized in high temperature deposition processes such as vacuum evaporation and sputtering.
In summary, there is a need within the field of x-ray optics for free-standing, thin films of polyimide. The films must be flat and contain no pinholes. Because the extent to which energy is absorbed by the filter depends critically on thickness, a feasible fabrication process must be capable of controlling film thickness to within .+-.10 percent of the nominal value, or 100 .ANG., whichever is the lessor of the two.
Heretofore, free-standing thin polyimide films have been fabricated by a number of methods, including: 1) casting a polyamic acid film onto the surface of a liquid bath, allowing the solvent to evaporate thus solidifying the film, and then recovering the film from the surface of the bath, 2) depositing a release agent onto a solid substrate, casting a polyamic acid film onto the substrate, and then floating the film off onto the surface of a liquid bath by dissolving away the release agent from beneath the film, 3) adhering a frame to a polyimide film while the film is still attached to the substrate onto which it was deposited, immersing all or a portion of the film, frame, and substrate assembly in a reactive chemical solution such as concentrated hydrofluoric acid to loosen the bond between the film and the substrate, and then removing the frame-supported film from the substrate, 4) depositing a release agent onto a solid substrate before the deposition of a polyimide film, adhering a frame to the film, and then removing the frame-supported film from the substrate by dissolving away the release agent from beneath the film, and 5) first removing a polyimide film from the substrate onto which it was initially deposited, said removal step being performed by air jets, liquid jets, or some other detachment means, and then attaching a frame to that free-standing film.
In U.S. Pat. No. 4,929,405 (issued May 29, 1990), R. S. Kohn fabricated a free-standing polyimide film by the first of those methods, and disclosed a process in which an ultrathin film was recovered by lifting the film from the surface of a liquid bath with the use of a 2" by 3" aluminium plate having a 3 cm diameter hole at its center. A difficulty inherent with this method is that the surface of the liquid bath must not be vibrated or the nascent thin film will be damaged. Another difficulty inherent with this method is that the surface of the liquid bath can, potentially, accumulate particulate contamination with the possibility of causing further damage to the nascent polymer film. Yet another difficulty with this method is that the final film thickness can be regulated only by adjusting the concentration of solids in the casting solution, since the film thickness is influenced by the extent to which the polyamic acid solution spreads out over the surface of the liquid bath. Still another difficulty with the method of U.S. Pat. No. 4,929,405 is that the "thermal treatment rendering the film insoluable in its casting solvent," i.e., the imidization step, is performed after the film has been attached to the aluminium plate, subjecting the film to a biaxial stress state as a result of the mismatch between the thermal expansion coefficients of the polyimide and the aluminium, respectively. Finally, the end uses of the polyimide films disclosed by Kohn that were 400 .ANG. or less in thickness involved the controlled release of drugs and the separation of gases, and did not include applications to x-ray optics for which thicknesses in the range 200 .ANG. to five microns would be useful.
In U.S. Pat. No. 2,631,334, J. S. Bailey teaches that a thin film can be recovered from the surface of a liquid bath with an adherent frame, but the fabrication process again involved casting the polymeric precursor solution onto the surface of a liquid bath, and the films that Bailey disclosed were made of nylon and not polyimide.
In U.S. Pat. No. 4,393,113 (issued Jul. 12, 1983), K. Sugie, T. Yamada, and T. Yamaji published a process for fabricating an ultrathin silicon containing copolymer membrane, but again the method involved casting the polymeric precursor solution onto the surface of a liquid bath. In this technique the film was recovered by a porous sheet-like material. The utility was concerned with the separation of gases.
In U.S. Pat. No. 4,374,891 (issued Feb. 22, 1983), W. J. Ward, III, specified a process for fabricating ultrathin films of a polyphenylene oxide/organopolysiloxane-polycarbonate copolymers, but again the method involved casting the polymeric precursor solution onto the surface of a liquid bath. Removal of the film from the surface of the casting substrate was accomplished by the use of a vacuum pickup on a microporous substrate. The applications of these films were stated to be gas separation devices.
In U.S. Pat. No. 4,155,793 (issued May 22, 1979), R. M. Salemme and W. R. Browall described the fabrication of ultrathin films of polymers using a continuous process, in which a flexible microporous support layer was fed into a liquid bath on whose surface resided the ultrathin polymer film. Again, the method for preparing the ultrathin film involved casting the polymeric precursor solution onto the surface of a liquid substrate.
A method in which metal films are floated off their substrate of deposition is discussed in an article entitled "The Preparation and Use of Unbacked Metal Films as Filters in the Extreme Ultraviolet," by W. R. Hunter, in Physics of Thin Films, Vol. 7, pp. 74-77 (1973). A release agent was deposited onto the substrate before the metal film was coated. The substrate was subsequently immersed in a liquid which served to dissolve the release agent beneath the metal film. This allowed the film to float off the substrate onto the surface of the liquid bath. The film was then recovered onto a frame. A similar floating technique was used to produce free-standing films of polyimide by P. Maier-Komor and was described in "Large Area Polyimide Foils for Heavy Ion Gas Detector Systems," Nuclear Instruments and Methods in Physics Research, A282, pp. 172-175 (1989).
A method in which a frame (or "carrier element") is attached to a polymer film before the film is released from the substrate onto which it was deposited is disclosed by J. F. Nester in U.S. Pat. No. 4,246,054 (issued Jan. 20, 1981). After the frame is attached to the film in Nester's technique, the edges of the frame, film, and substrate assembly are immersed in a chemical solution designed to react with and loosen the bond between the film and the substrate. An example of an appropriate chemical solution described by Nester is concentrated hydrofluoric acid. The entire assembly is then, in a separate step, immersed completely in water to cause the substrate to fall away from the frame-supported film. It should be noted that the edges of the film, frame, and substrate assembly in Nester's method are immersed in the reactive chemical solution prior to insertion of the assembly in the water bath, and that immersion in the reactive chemical solution represents a distinct and separate step from that of immersion in the water bath. An obvious difficulty with this method is that exposing the thin polymer film to a harsh chemical substance such as concentrated hydrofluoric acid has the potential to damage the film. Additionally, the manner in which the film, frame, and substrate assembly is immersed completely in water in the second, water bath immersion step necessarily exposes the film that is destined to become the final product to contaminants in the water bath. The utility of Nester's free-standing polymer membranes relates to x-ray lithography masks.
In U.S. Pat. No. 4,536,240 (issued Aug. 20, 1985), R. Winn also discloses a method for making frame-supported, free-standing membranes by means of attaching an adherent frame to the film before the film is released from the substrate (or "forming surface"), but only in the case when a parting or release agent has been used. In this embodiment a parting or release agent is applied to the substrate before the polymer film is deposited. The release agent is dissolved away from beneath the film to cause the frame-mounted film to separate from the substrate. A difficulty inherent with this method is that a release agent can damage and/or contaminate the polymeric film. Winn describes a second technique in which the film is first removed from the substrate by air jets, liquid jets, or other detachment means, and the film is attached to a frame in a subsequent step. An obvious difficulty with this technique is that detachment means such as air jets and liquid jets have the potential to cause damage to a thin polymeric film. Winn's membranes were stated to have optical applications.
In summary, the prior art teaches that the bond between a polyimide film and the substrate onto which it was deposited has an adhesive strength large enough to cause difficulties when fabricating a free-standing version of the film. One method of fabricating free-standing films involves avoiding a solid substrate of deposition entirely, as in the case when a film is cast onto the surface of a liquid bath. When a solid substrate is used, the prior art teaches that chemical solutions, parting agents, release agents, air jets, mechanical jets, and other similar detachment means must be employed to cause separation of the film from the substrate.